This invention relates in general to high speed printing systems for use in conjunction with computers. More specifically, it relates to a mechanical arrangement for selecting cut sheet paper from only one of several cassettes each of which may hold a supply of a differently sized paper.
In many printing applications, particularly office printers used as the output device of a word processing system or other host data source, it is highly desirable to be able to print on cut sheet paper products having a variety of sizes or other characteristics. For example, it is frequently desirable to be able to print on standard letter size paper, legal size paper, or envelopes. Usually each of these products is held in a separate cassette and all of the cassettes can feed their contents to the printhead of the printer. It is also known to employ a feed roll in the cassette which strips the uppermost sheet from the supply and directs it into a common paper feed arrangement leading to the printhead. This approach requires, however, that there be some arrangement for feeding paper from only a selected one of these cassettes.
One arrangement in the prior art has been to move the entire set of cassettes relative to a single, fixed feed roller until the appropriate cassette is positioned for feeding its paper to the printer. This arrangement has certain time and cost disadvantages associated with moving a comparatively large assembly in a precise manner. There are also design difficulties in engaging and disengaging the feed roller from the paper supply held in the selected cassette. Another approach, where there are only two cassettes, has been to mechanically switch a common feed roller between the two cassettes. This arrangement has the obvious limitation that it is only useful for the feeding one of two types of cut sheet paper. It is also comparatively complex mechanically.
The most commonly used selection system is to electrically activate the feed roll of the selected cassette. U.S. Pat. No. 3,936,042, for example, selectively engages feed rolls of the cassettes through a magnetic clutch. U.S. Pat. No. 4,258,607 also shows a system of this general type. The feed rolls of each cassette can be driven by a separate motor and controlled electrically by the machine or data source controlling the printing operation. Alternatively, a common conveyor belt for the paper can engage friction wheels associated with each cassette which are in turn selectively coupled to the cassette feed rolls. This '607 patent, however, offers no details as to how one would achieve this coupling other than through the obvious expedient of separate, electrically operated clutches. In general, electrical cassette selection systems of this type are relatively costly to manufacture and more cumbersome to install and use than a mechanical system. For example, such a system may require multiple motors or clutches and a significant redesign of existing cut sheet printers. In use, electrical selection systems may require an operator to make electrical connections when a cassette is replaced or resupplied or require multiple controls for the printer and feed system.
Ideally, the cassettes should also be easily replaceable with a simple sliding insertion or removal to facilitate the selection of different types of paper or the replenishment of the paper supply held in the cassette. The paper feed system should also be compatible with known printers, and preferably should involve a minimum modification of existing systems. Related to the foregoing, it is desirable that all of the cassettes have a common drive mechanism. Also, of course, it is important that the system feed paper reliably and shift from one cassette to another during a printing operation with a minimum delay. While these design considerations are well known in the industry, no purely mechanical paper feed system heretofore has been able to simultaneously satisfy all of these criteria.
It is therefore a principal object of this invention to provide a paper feed system having multiple cassettes each holding a different type of cut sheet paper which reliably feeds paper from only a selected one of the cassettes with no electrical connection between the printer and the feeder.
Another object of the invention is to provide a paper feed system where the selection mechanism is programmable to provide a high degree of flexibility and yet is purely mechanical in nature.
A further object of this invention is to provide a paper feed system with the foregoing advantages where there is a common drive for all of the cassettes.
A further object of this invention is to provide a paper feed system where the cassettes are readily replaceable through a simple slip fit insertion that also engages and disengages the paper feed mechanism of each cassette from the drive system of the paper feed portion of the printer.
Still another object of the invention is to provide a multiple cassette paper feed system for a printer that is compact, comparatively cost effective, and requires a minimum modification of existing printers of this type to accommodate the selection system of the present invention.